The character of Carol Aird was inspired by Virginia Kent Catherwood (1915-1966), a Philadelphia socialite six years older than Patricia Highsmith with whom the author had a love affair in the 1940s. Catherwood lost custody of her daughter after her homosexuality was used against her with a taped recording of a lesbian liaison she had in a hotel room. ("'Instantly, I love her': the affairs that inspired Carol". The Telegraph, 28 November 2015)
Although Cate Blanchett and Rooney Mara weren't required to be on the other end of the line whenever they talk on the phone, they offered to do so to help each other out. Thus whenever Carol and Therese talk to each other on the phone, Blanchett and Mara are really on the other end of the line.
Rooney Mara has been a big fan of Cate Blanchett since she was 13 years old and described the experience working with Blanchett as "beyond a dream come true."
The novel "The Price of Salt" was inspired by a blonde woman in a mink coat (Mrs. E.R. Senn, née Kathleen Wiggins) who ordered a doll from Patricia Highsmith when she was working as a temporary salesgirl in the toy section of Bloomingdale's in New York City during the 1948 Christmas season. Highsmith recalled feeling "odd and swimmy in the head, near to fainting, yet at the same time uplifted, as if I had seen a vision." She completed the outline for the story in about two hours that same night, likely under the influence of chicken pox which she discovered she had the next day. Highsmith wrote in the Afterword for the 1990 new edition of the novel: "One of the small runny-nosed children there must have passed on the germ, but in a way the germ of a book too: fever is stimulating to the imagination." She completed "The Price of Salt" by 1951.
Carol was shot on Super 16 millimeter film to resemble the look and feel of photographic film from the late 1940s/early 1950s. The cinematography was influenced by the photojournalism of Vivian Maier, Ruth Orkin, Helen Levitt, and Esther Bubley. Photography by Saul Leiter (known for shooting through windows and using reflection) was a key influence.